DIRECT AND SEQUENTIAL POTENTIOMETRIC DETERMINATION OF HYPOCHLORITE, CHLORITE, AND CHLORATE IONS WHEN HYPOCHLORITE ION IS PRESENT IN LARGE EXCESS

A selective potentiometric technique has been developed for the determination of OCl- at high concentration (e.g., 1-3 M) and ClO2- and ClO3- at comparatively low concentrations (e.g., 50-150 mg/L) when these species are present together in solution. The method uses sulfite ion as a mask to selectively and quantitatively remove OCl- at pH 10.5, Thus, OCl- is measured and masked at the same time. At pH 10.5, SO32- does not react with ClO2- or ClO3-. The remaining SO32- is quantitatively removed with triiodide ion, Following the masking procedure, the sequential determination of ClO2- and ClO3- is carried out either by iodometric titration at the appropriate pH or by ion chromatography, The results from the direct potentiometric titration using SO32- are compared to those of an indirect reference method.